A circular annular seal ring made from a resin is adapted to be disposed between an outer member and an inner member that rotate relative to each other. The seal ring is disposed in a circumferential groove of the outer member or the inner member to isolate a liquid space from an external space. The seal ring is formed from an elongated and arcuate rod to define a circle by joining ends of the rod, and has a junction formed from the ends. The seal ring has a radial thickness gradually decreasing from two points that are apart from the junction toward the junction. The points are separated from the junction by an angle of not less than 60 degrees and not more than 150 degrees about a central axis of the circle.